1. Field of the Invention
This invention relates generally to the field of semiconductor memory, sensor applications and logic devices. The invention relates more specifically to a multilayer dielectric tunnel barrier used in magnetic tunnel junction devices, and its method of fabrication.
2. Description of the Related Art
Recently, magnetoresistive random access memory (MRAM) devices have been developed as possible use for data storage. The nonvolatility, fast access times, and less complex structure of MRAM offers some advantages over DRAM and FLASH memory devices.
The two most critical layers in an MRAM memory cell are the tunnel barrier, which is often formed with AlxOy i.e., Al2O3, and the sense (free) layer. The tunnel barrier is very thin and the tunneling resistance is exponentially dependent on its thickness. This strong dependence on thickness makes it difficult to consistently produce devices with nearly the same resistance over typical substrate sizes currently used in memory manufacturing processes.
A magnetic tunnel junction device, in its simplest form, is two ferromagnetic layers separated by the tunnel barrier or dielectric film. The two ferromagnetic layers, together with the tunnel barrier, act to pass certain electrons preferentially based on their respective spins. One ferromagnetic layer has a pinned magnetic field whereas the other ferromagnetic layer has a magnetic field which freely switches directions based on the applied magnetic programming signal.
For proper device operation, the tunnel barrier must be free of pinholes, very smooth, and uniform over the entire wafer. Small variations in the thickness of the tunnel barrier layer such as over the surface of a wafer, can result in large variations in memory cell resistance. Typically, an AlxOy tunnel barrier layer is fabricated by depositing a metallic aluminum layer and subsequently oxiding this layer by one of several methods. Oxidation can occur by plasma oxidation, oxidation in air, oxidation by glow-discharge plasma, atomic-oxygen exposure, and ultraviolet-stimulated oxygen exposure.
However, the oxidation process creates anomalous effects. Overoxidation or underoxidation occurring on the aluminum layer reduces the magnetoresistance ratio. The magnetoresistance ratio is typically the change of resistance proportional to the square of the magnetic field. Overoxidation results in oxidation of the magnetic electrode beneath the tunnel barrier; whereas, underoxidation leaves metallic aluminum at the bottom of the tunnel barrier. In addition, roughness at the tunnel barrier interfaces lowers the magnetoresistance ratio dramatically, due to partial shorts or tunneling hot spots. Thus, producing a magnetic tunnel junction material with good resistance uniformity over an entire wafer is challenging. Many techniques have been employed to improve the aluminum metal layer thickness uniformity, such as forming the AlxOy tunnel barrier with air, reactive sputtering, plasma oxidation with plasma source, plasma oxidation with power introduced from the target side, and plasma oxidation with power introduced from the substrate side.
Another problem arises when the magnetic tunnel junction material is exposed to temperatures greater than 300° C. The magnetoresistance ratio begins to degrade at 300° C. and drops off sharply at 400° C. as a result of increasing resistance. In addition, as bit sizes are reduced, a challenge arises in producing magnetic tunnel junction material with very low resistance-area products. The resistance of the magnetic tunnel junction material, expressed as the resistance-area, has been shown to vary exponentially with both the metal layer's thickness, oxidation dose for thickness, and dose values that produce a high magnetoresistance ratio. Thus, obtaining a thinner tunnel barrier without reducing the magnetoresistance ratio is one of the key factors to achieving a low resistance-area product or device.
Accordingly, a need exists for an improved tunnel barrier film which has lower resistance and in which the tunnel barrier height and final overall barrier resistance can be modified. Additional needs exist for an improved tunnel barrier film with reduced chances of pinhole formation and which can work in temperatures greater than 300° C. An additional need also exists for a tunnel barrier film having reduced roughness at the tunnel-barrier interfaces and which mitigates oxidation problems.